Backlighting a television, computer, or other display allows a user to view the display in a dimly lit environment. Some current backlighting configurations use strings of light emitting diodes. FIGS. 1 and 2 show two such prior art configurations.
FIG. 1 shows a backlighting configuration 100 that includes two LED strings 101A and 101B, powered through a transformer 150. The remainder of the circuitry, described below, is used to turn the LED strings 101A and 101B ON and OFF, thereby controlling the brightness of the backlighting. Generally, a display will include more than two LED strings. FIGS. 1 and 2 both show only two LED strings merely to simplify the drawings.
In operation, the LED string 101A is illuminated by a current flowing through it. That current is determined by a voltage difference ΔV between its input and output. Transistor 105A is used to turn ON and OFF the LED string 101A and when transistor 105A is ON, the voltage at the bottom of the LED string 101A is approximately the same as the voltage at the top of resistor 110A. LED strings have the same brightness when they have the same current, but the voltage across the strings will vary from string to string. The regulator loop 120A regulates the current in the string 101A by monitoring the voltage on sense resistor 110A and then adjusting the voltage at the top of the string 101A until it is at the correct value for the string 101A to be at the correct current. In the example of FIG. 1, string 101A has ΔV=(220−VA) VDC while string 101B has ΔV=(210−VB) VDC, but the string currents are the same. The value of the resistor 110A determines the current through and thus the full brightness of the LED string 101A. During a single cycle, the longer the transistor 105A is ON, allowing current to flow through the LED string 101A, the brighter the effective illumination of the LED string 101A. For example, a 25% duty cycle results in the LED string 101A being illuminated to 25% of its perceived full brightness. A pulse width modulation (PWM) signal input to the transistor 105A controls the duration of the current through the LED string 101A, thereby adjusting its perceived brightness.
The control circuit 120A is part of a feedback loop for a Boost Regulator used to control the illumination of the LED string 101A. The control circuit 120A monitors the voltage drop on the sense resistor 110A and thus the current through the LED string 101A. The control circuit 120A is part of a Boost Regulator that controls the first voltage by adjusting a PWM duty cycle at the gate of the transistor 125A. This PWM frequency at the transistor 125A is independent of the PWM signal on the transistor 105A and is typically 1000 times higher in frequency.
To ensure sufficient voltage, the LED string 101A is coupled to a “boost circuit,” which includes an inductor 135A, transistor 125A, and diode 130A that together boost the 115 VDC voltage from the top rail of a transformer secondary to produce the 220 VDC. The voltage divider 132A is used to monitor the Boost converter output in case of a fault condition such as an open LED string that would result in an over voltage condition.
FIG. 1 also shows other components on a non-isolated side of the transformer that powers the backlighting configuration 100, components such as a Bridge and Power Factor Correction element 161, a controller 165, and an opto-coupler 169. Because these components are ancillary to this invention, they will not be discussed here.
FIG. 2 shows a backlighting configuration 200 that differs slightly from the configuration 100. Whereas the configuration 100 uses multiple boost circuits (e.g., elements 125A, 130A and 135A, and elements 125B, 130B and 135B) each powering a separate LED string, the configuration 200 uses a single boost circuit (elements 125, 130 and 135) to power all the LED strings. In the configuration 200, the voltage at the bottom of the LED string 101A is controlled by adjusting the voltage at the gate of the transistor 105A.
It will be appreciated by those of ordinary skill in the art that multiple LED strings are typically used to backlight a display. For example, one LED string can be placed on each edge of a display. Alternatively, two LED strings can be placed along an edge of a display, such as a widescreen display.
The configurations 100 and 200 have many of the same components. Referring to FIGS. 1 and 2, identical labels refer to the same element. The figures are also simplified. For example, in FIG. 2 dots are shown to illustrate that some elements (e.g., additional LED strings and a controller similar to the amplifier 115A) are not shown.
The configurations 100 and 200 share several disadvantages. They both use high-voltage boost circuits, low-frequency inductors, and linear regulators, components that generate a lot of heat, are relatively expensive, and take up a large portion of a printed circuit board. Any mismatches between the diodes in the LED strings result in voltage differences across each current source, generating even more heat. Because high voltages are used, the regulation loop bandwidth is relatively limited.
Moreover, because the regulator circuit for controlling the current through each LED string is coupled to the high-voltage input, the components are required to withstand high-voltage conditions. As such, conventional controllers for backlight circuits are large and expensive.